3rd generation partnership project (3GPP) long term evolution (LTE) is an improved version of a universal mobile telecommunication system (UMTS) and is introduced as the 3GPP release 8. The 3GPP LTE uses orthogonal frequency division multiple access (OFDMA) in a downlink, and uses single carrier-frequency division multiple access (SC-FDMA) in an uplink. The 3GPP LTE employs multiple input multiple output (MIMO) having up to four antennas. In recent years, there is an ongoing discussion on 3GPP LTE-advanced (LTE-A) that is an evolution of the 3GPP LTE.
The 3GPP LTE-A employs various techniques such as carrier aggregation, relay, etc. The 3GPP LTE system is a single carrier system supporting only one bandwidth (i.e., one component carrier) among {1.4, 3, 5, 10, 15, 20} MHz. On the other hand, the LTE-A employs multiple carriers using carrier aggregation. The component carrier is defined with a center frequency and a bandwidth. The component carrier may correspond to one cell. A multiple carrier system uses a plurality of component carriers having a narrower bandwidth than a full bandwidth.
To decrease interference caused by uplink transmission between user equipments (UEs), it is important for a base station (BS) to maintain uplink time alignment of the UEs. The UE may be located in any area in a cell. An uplink signal transmitted by the UE may arrive to the BS at a different time according to the location of the UE. A signal arrival time of a UE located in a cell edge is longer than a signal arrival time of a UE located in a cell center. On the contrary, the signal arrival time of the UE located in the cell center is shorter than the signal arrival time of the UE located in the cell edge.
To decrease interference between the UEs, the BS needs to performing scheduling so that uplink signals transmitted by the UEs in the cell can be received within a boundary every time. The BS has to properly regulate transmission timing of each UE according to a situation of each UE. Such a regulation is called maintenance of time alignment.
As multiple carriers are introduced, uplink time alignment needs to be maintained in each component carrier (or serving cell). A signaling overhead for maintaining the uplink time alignment may increase in proportion to the number of component carriers.
Accordingly, there is a need for a method of maintaining uplink time alignment for a plurality of component carriers.